By means of radiometric measuring devices, physical variables, such as e.g. a fill level of a fill substance in a container, an exceeding or subceeding of a predetermined fill level of a fill substance in a container, or a density of a medium, are measurable.
Radiometric measuring devices are applied, usually, when conventional measuring devices are not applicable due to especially difficult conditions at the measuring location. Very frequently, there reigns at the measuring location e.g. extremely high temperatures and pressures or chemically and/or mechanically very aggressive environmental influences, which make the use of other measuring methods impossible.
In radiometric measurements, a radioactive radiator, e.g. a Co 60 or Cs 137 preparation, is placed in a radiation protection container at a measuring location, e.g. a container containing a fill substance. Such a container can be e.g. a tank, a pipe, a conveyor belt or any other form of Containment.
The radiation protection container includes a passageway, through which radiation emitted from the radiator positioned for measurement is radiated through a wall of the radiation protection container.
Usually, a radiation direction is selected, such that the radiation penetrates that region of the container, which should be scanned for measurements reasons. On the oppositely lying side, the emerging radiation intensity changed by a fill level, or density, change is quantitatively registered with a detector. The emerging radiation intensity depends on the geometric arrangement and on absorption. The latter is, in the case of fill level measurement and in the case of monitoring of an exceeding, or subceeding (falling beneath), of a predetermined fill level, dependent on the amount of the fill substance in the container and, in the case of the density measurement, on the density of the fill substance. As a result, the emerging radiation intensity is a measure for the current fill level, the exceeding, or subceeding (falling beneath), of the predetermined fill level, or the current density of the fill substance in the container.
Suited as detector is e.g. a scintillation detector having a scintillator, e.g. a scintillation rod, and a photomultiplier. The scintillation rod is composed of a special synthetic material, such as e.g. polystyrene (PS) or polyvinyl toluene (PVT), which is optically very pure. Under the influence of gamma radiation, light flashes are emitted by the scintillation material. These are registered by the photomultiplier and converted into electrical pulses. A pulse rate, with which the pulses occur, depends on the radiation intensity and is, thus, a measure for the physical variable to be measured, e.g. the fill level or the density. Scintillator and photomultiplier are usually mounted in a protective tube, e.g. of stainless steel.
The measuring device includes, associated with the detector, a measuring device electronics, which produces an output signal corresponding to the pulse rate. The measuring device electronics comprises, usually, a controller and a counter. The electrical pulses are counted and a counting rate derived, on the basis of which the physical variable to be measured is ascertainable. The ascertaining of the measured variable occurs, for example, by means of a microprocessor provided in the electronics and is made available by the measuring device in the form of a measurement signal. The measurement signal is, for example, supplied to a superordinated unit, e.g. a programmable logic controller (PLC), a process control system (PCS) or a personal computer (PC).
In measuring, and control, technology, preferably measuring devices with only one line pair are applied, via which both the supplying of the measuring device with energy, or power, as well as also the signal transmission occurs. These devices are frequently referred to as 2-wire, measuring devices.
According to standard, such measuring devices are supplied with 10 V to 12 V and the measuring device controls an electrical current flowing through the line-pair as a function of an instantaneous, measured value. The measurement signal is, in the case of these measuring devices, an electrical current. In a standard, which is usual in measuring, and control, technology, the electrical current is set as a function of the instantaneous measured value to values between a minimum electrical current of 4 mA and a maximum electrical current of 20 mA. These devices offer the advantage, that, due to the small energy supply, they can be used also in explosion endangered areas, where an intrinsically safe, electrical current supply is required.
Since both the supplying of energy, or power, as well as also signal transmission occurs via the line-pair, the measuring device has available, in the case of a supply voltage of 12 V and an electrical current of 4 mA, a power of only 48 mW.
In the case of another variant of these 2-wire measuring devices, the devices are connected via a bus, via which both the supplying of the measuring device with energy, or power, as well as also the signal transmission occurs. Also for this variant, corresponding industrial standards have become common, such as e.g. the Profibus and Foundation Fieldbus standards. Also in the case of these 2-wire, bus devices, as a rule, only very little energy is available for operating the measuring device. Typically, the terminal voltage here amounts to 10 V and an average electrical current of 9 mA flows. The available power lies therewith around 90 mW.
Conventional radiometric measuring devices require, however, especially for supplying the photomultiplier with high voltage, very much more energy than is available for 2-wire measuring devices.
For operating the photomultiplier, a high voltage of up to 2000 V is required. Usually, this high voltage is produced by means of a DC/DC converter and distributed via a voltage divider, e.g. a resistance chain, to the individual dynodes of the photomultiplier. For this purpose, preferably very high ohm, voltage dividers are applied. However, even in such case, cross-current flows through the voltage divider, which, compared to the actual electrical current requirement of the photomultiplier, leads to considerable energy losses.
In order that these measuring devices, in spite of this, can be used in connection with the earlier described standards, these measuring devices, usually, have two line-pairs. Via one of the line-pairs, the measuring device is supplied with energy, or power, and via the other there flows an electrical current corresponding to the earlier described standard. For the power supply, it is usually required, in the case of the normal electrical supply line delivering e.g. 230 V alternating voltage, to provide a transformer and a rectifier, in order to obtain e.g. a supply voltage of, usually, 24 V direct voltage for the measuring device. This is very complicated and there is danger that the two line-pairs can be switched, one for the other, in the connecting of the device.
There are also radiometric measuring devices on the market, in the case of which the detector and the associated measuring device electronics are elements separated from one another, which are, in such case, supplied with energy separately from one another.